An electro-rheological fluid is a slurry of finely-divided hydrophilic solids suspended in a hydrophobic liquid. The flow properties of such a slurry are dependent upon the strength of the electric field to which it is subjected. Up to a transition value of electric field the slurry behaves as a simple Newtonian liquid: for electric fields greater than the transition value, but less than that required to cause electrical breakdown, the fluid behaves approximately as a Bingham plastic, that is, no flow whatsoever is caused by shear stresses less than a yield stress which is dependent upon the fluid and on the applied field.
In many of the applications proposed for these fluids there is a requirement for either a pump capable of pumping an electro-rheological fluid around a fluid circuit or a rotary actuator powered by a supply of electro-rheological fluid under pressure. Preferably the device employed should not rely for its operation on close-fitting components, such as valve seats, that may become clogged by accumulations of solid particles coming out of suspension nor rely on intermeshing components, such as gears, which tend to grind the solids of the slurry thereby increasing the fluid's no-field viscosity.